1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, to a light modulator-semiconductor laser device, and a semiconductor laser.
2. Background Art
Recently, a light modulator-semiconductor laser device formed by combining a light modulator and a semiconductor laser has been practically applied to an optical communication system.
FIG. 9 is a perspective view of a conventional light modulator-semiconductor laser device. As shown in FIG. 9, the light modulator-semiconductor laser device includes a semiconductor laser unit 30 and a light modulator unit 31.
FIG. 10 is a sectional view of the semiconductor laser unit 30 taken along a line perpendicular to an optical axis 32 in FIG. 9. Referring to FIG. 10, the semiconductor laser unit 30 includes an n-InP substrate 1, a cathode 2 of Ti/Au, an anode 3 of Ti/Au, an insulating film 5 of silicon dioxide (SiO2) or silicon nitride (SiN), an n-InP clad layer 6, and an InGaAsP multiple quantum well active layer 7. The light modulator unit 31 is provided with an InGaAsP multiple quantum well absorbing layer at a position corresponding to the InGaAsP multiple quantum well active layer 7, and an anode 4 at a position corresponding to the anode 3.
Shown also in FIG. 10 are a p-InP first clad layer 8, a p-InGaAsP diffraction grating 9, a p-InP second clad layer 10, a p-InP first buried layer 11, an n-InP second buried layer 12, a p-InP third buried layer 13, a p-InP third clad layer 14, and a p-InGaAsP contact layer 15.
The operation of the conventional light modulator-semiconductor laser device will be described. When a forward current flows through the anode 3 and the cathode 2 of the semiconductor laser unit 30, stimulated emission of light occurs in the InGaAsP multiple quantum well active layer 7 and laser oscillation of a frequency dependent on the grating constant of the p-InGaAsP diffraction grating 9 occurs. The p-InP first buried layer 11, the n-InP second buried layer 12 and the p-InP third buried layer 13 have a current constricting function to inject a current efficiently into the InGaAsP multiple quantum well active layer 7.
When a voltage is applied across the anode 4 and the cathode 2 of the light modulator unit 31 to reverse-bias the pn-junction and a high-frequency signal is superposed on the voltage, the quantum-confined Stark effect of the quantum well and the Franz-Keldysh effect of the semiconductor change a light absorption spectrum. When a light beam of a wavelength emitted by the semiconductor laser unit 30 falls on the light modulator unit 31, the intensity of the light beam is modulated by the high-frequency signal.
The light modulator-semiconductor laser device is capable of producing an intensity-modulated optical signal and is used as a light source for an optical transmitter.
The light modulator-semiconductor laser device modulates the light beam emitted by the semiconductor laser unit 30 in the light modulator unit 31. Therefore, the light modulator unit 31 must be contiguous with the semiconductor laser unit 30. Electromagnetic waves generated by the light modulator unit 31 when a high-frequency voltage is applied to the light modulator unit 31 for modulation propagate through space and part of the electromagnetic waves enter the semiconductor laser unit 30.
Carrier density in the InGaAsP multiple quantum well active layer 7 varies with time when a high-frequency current is supplied to the semiconductor laser unit 30 and, consequently, the refractive index of the InGaAsP multiple quantum well active layer 7 varies. Thus, the wavelength of the light beam incident on the light modulator unit 31 is modulated and the width of the spectrum of the modulated light increases unnecessarily. Consequently, the transmission distance of an optical fiber having a large chromatic dispersion is reduced, causing the deterioration of the characteristic of the optical transmitter.
The present invention has been made to solve the foregoing problems and it is therefore an object of the present invention to provide a semiconductor device capable of preventing the leakage of a high-frequency current into a semiconductor laser when a high-frequency voltage is applied to a light modulator and of enabling high-speed optical transmission, and a method of manufacturing such a semiconductor device.
According to one aspect of the present invention, there is provided a semiconductor device having an active layer, a first semiconductor layer formed on one side of the active layer and a second semiconductor layer formed on the other side of the active layer, and capable of emitting light when a voltage is applied across the first and the second semiconductor layer. The semiconductor device comprises a first electrode formed on the first semiconductor layer, to which a predetermined voltage is applied, and a second electrode formed on an insulating film so as to cover at least part of the first electrode. And the second electrode is grounded.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device having an active layer, a first semiconductor layer formed on one side of the active layer and a second semiconductor layer formed on the other side of the active layer, and capable of emitting light when a voltage is applied across the first and the second semiconductor layer. The method comprises the following steps. Firstly the first semiconductor layer is formed on a semiconductor substrate. Secondly the active layer is formed on the first semiconductor layer. Thirdly the second semiconductor layer is formed on the active layer. Fourthly a laminated film consisting of the second semiconductor layer, the active layer and the first semiconductor layer is selectively etched in a ridge such that openings reaching the semiconductor substrate are formed on the opposite sides of the ridge. Fifthly a first conductive film is formed on the second semiconductor layer. Sixthly an insulating film is formed on the first conductive film. Seventhly a second conductive film is formed on the insulating film so as to cover the first conductive film and is connected through the openings to the semiconductor substrate.
According to the present invention, since the second electrode is formed on the insulating film covering the first electrode and the second electrode is grounded, the first electrode to which a predetermined voltage is applied can be screened and hence, even if electromagnetic waves propagate through space, the flow of leakage current between both the semiconductor layers due to the effect of the electromagnetic waves can be prevented.
Other and further objects, features and advantages of the invention will appear more fully from the following description.